Project Background

Plant invasion-fire regimes in deserts

Deserts tend to experience less fire than other biomes because water and nutrient limitations to plant growth result in low and discontinuous fuel loads. However, recent analyses shows that deserts of North America may be at a “tipping point” in which fires that historically were rare and patchy are increasing in frequency, size and intensity. For example, just a few large scale fires burned more than 318, 000 ha of Mojave Desert shrubland in 2005 alone. Similarly, large areas in the Great Basin Desert have burned in recent years and it is estimated that fire return intervals that occurred on century time scales historically, can now have a periodicity of a decade or less.

There is evidence that dramatic expansion of invasive annual grasses (Bromus and Schismus spp.) under climate conditions that are still poorly defined underlie increases in fire size and frequency. Changing fire regimes have the potential to alter the functional integrity of desert ecosystems since desert biota tend to be poorly adapted to fire. Desert fire destabilizes soil crusts and results in pulses of soil nitrogen that further promotes invasive grass establishment. There is concern that the emergence of invasive grass-fire regimes with shorter fire intervals may promote state changes in vegetation that can fundamentally alter functional interactions biological community with unknown ecological consequences. Conversion of native desert shrub communities to invasive annual grasslands can shift deserts from carbon sinks to carbon sources and alter surface albedo with strong potential feedbacks on regional climate patterns. Dust transport out of fire destabilized deserts and subsequent deposition on Rocky Mountain snowpack dramatically increases melt rates, which has adverse effects on water yields for urban and agricultural use in western states that are already water limited and alters hydrological balance in montane forest communities.

Controls of precipitation and soil characteristics on invasive plant growth and fire

Understanding the effects of climate and soil characteristics on plant growth is paramount to identifying the behavior and consequences of fire in desert landscapes. Precipitation patterns largely control the temporal distribution of desert plants, while soil attributes strongly influence their spatial distribution. Soil moisture is the primary limiting factor for germination and growth of annual desert plants including invasive grasses that carry fire. While it has long been understood that the quantity of precipitation is an important driver of plant productivity and function, it is increasingly becoming apparent that the distribution of precipitation (frequency, timing) also matters. However, climate change is expected to increase variability in precipitation by altering the frequency and intensity of rain and drought events. For desert communities of western North America, current climate projections predict higher amounts of fall/winter precipitation, decreased spring/summer rain and greater frequency of extreme weather events (drought and wet periods). There is evidence that vegetation in arid systems will be particularly sensitive to these shifts in precipitation patterns (34). What is lacking is an understanding of how changing precipitation patterns influence invasive grass growth that fuels desert fire, and how it affects the post-fire recovery potential of native plant communities.

Heterogeneity in soil parent materials, topographical conditions, plant community characteristics and transport processes result in high spatial variability in the physical and chemical properties of desert soils that also exert strong control on the growth potential of both native desert plants and invasive annuals.

Top-down controls

Desert plants are almost exclusively angiosperms that largely depend on insect vectors for pollination and small mammals for seed dispersal to successfully reproduce. Desert fire is altering the abundance and composition of small mammal communities (citations) but its effects on pollinators are largely unknown. Few if any studies have examined the effects of desert fire on the top down forces exerted by small mammals and insect pollinators on desert plant communities. Despite the widespread use of deserts in Western North America as rangelands and the rapidly increasing area that has burned, little is known about the effects of livestock grazing on plant community re-establishment following fire. There is an extensive literature on pollination, seed dispersal and herbivory in deserts, but relatively little research has explored these biological interactions in the context of the three most important emerging environmental forces shaping desert ecosystem today: climate change, plant invasions and fire